Unnatural selection: Wily weeds outwit herbicides

The weedkillers atrazine and simazine were introduced in 1958. Ten years later, a plant nursery in the US that had been regularly using the pesticides reported that they were no longer effective against a plant called common groundsel – the first confirmed case of herbicide resistance.

Half a century on, the number of known strains of resistant weeds stands at 357 and counting. "Herbicide resistance is a fantastic example of evolution in response to human-induced selection pressure," says Stephen Powles of the University of Western Australia in Perth, who studies the problem.

Because of its huge commercial importance, a lot of money is spent studying the problem and in many cases we know exactly how plants are evolving resistance. The mechanisms range from changes in leaf shape or waxiness to reduce herbicide uptake, to mutations that prevent herbicides binding to the proteins they target.

Strategies such as alternating the type of herbicide used can slow the evolution of resistance, but it is not foolproof. Many weeds have developed resistance to more than one herbicide. In some cases, this is due to plants evolving resistance mechanisms that are effective against more than one pesticide. For instance, many break down pesticides using new variants or higher levels of enzymes of a kind called P450s. These enzymes often protect against a range of different herbicides. In the 1980s, two weeds were found to be resistant to weedkillers that had never been used in the field. In other words, says Powles, weeds can evolve resistance to herbicides that have not even been developed yet.

Multiple resistance can also arise when different weed strains swap genes. In Australia, one kind of ryegrass weed, Lolium rigidum biotype VLR69, is immune to at least nine herbicides thanks to six separate resistance mechanisms. In many areas around the world, resistance to the most popular and environmentally friendly weedkiller, glyphosate, is becoming a serious problem. "As glyphosate fails, farmers will be forced to resort to more costly and more time-consuming practices to control their crop weeds," Powles says.

That could lead to higher food prices although, as Powles point out, world grain production and prices depend on a multitude of factors, including climate, biology and politics. "It is thus very difficult to quantify the impact that glyphosate resistance will have within this mix except to state that it will be a very negative impact!" he says.

If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.

Common groundsel, the first of many (Image: Nigel Cattlin/Visuals Unlimited/Getty)